In the television broadcasting industry, an attempt is being made to enhance high definition television (HDTV) systems to account for problems associated with broadcasting broadband signals over existing television frequency channels. Some of the problems encountered when broadcasting any signal over existing television frequencies include phase interference caused by multipath signal reception, signal fading, and signal attenuation caused by atmospheric conditions, terrain conditions, distance from the transmission source, and the like. These problems are exacerbated when an attempt is made to send and recover a digital signal in an error free manner, such as required by the current HDTV standard for non-degraded HDTV reception.
The current HDTV standard as promulgated by the Advanced Television Systems Committee (ATSC) employs many features to make use of the limited bandwidth, i.e., 6 MHz, available for HDTV transmission on air and in cable. For example, the HDTV baseband signal undergoes the known MPEG 2 compression, and a known type of forward error correction (FEC) Reed Solomon (RS) encoding in order to compress and error correct the data payload associated with HDTV transmission. The HDTV standard also calls for randomization of the bit stream by injection of a pseudorandom code to ensure that the signal is evenly distributed across the allocated spectrum, i.e., channel. As is known in the art, data interleaving is also implemented to scramble the sequential order of the data stream and to disperse the MPEG 2 packet data throughout time in order to minimize the transmitted signal's sensitivity to burst type interference. To further accommodate the 6 MHz allotted to the transmission channel, the interleaved HDTV bit stream of packets is further encoded to limit the number of amplitude levels required in transmission.
The current ATSC standard calls for vestigial sideband transmission with 8 discrete amplitude levels (8-VSB), i.e., channel symbols. One encoding method that implements additional FEC while achieving the 8-VSB requirement is known as the trellis code. Trellis codes, which are convolutional, i.e., serial and dependent on previous information bit values, are a known method of data encoding. The current ATSC specification calls for a 2/3 trellis code for 8-VSB. As is known, the 2/3 ratio specifies that the trellis code will encode 3 bits for every 2 bits input to the encoder, i.e., a 2/3 rate encoder is specified. According to the present state of the art, existing receivers are implemented to decode the 2/3 encoding of the specified 2/3 trellis encoder.
FIG. 1 illustrates an exemplary prior art HDTV transmission encoding system. As shown, a standard packet stream 110 is input into the system where the stream is first randomized by data randomizer block 120 and encoded by Reed Solomon (RS) encoder 130. For the reasons discussed above, the randomized and encoded packet stream is then interleaved by data interleaver 140. Trellis encoder 150, typically running at a 2/3 encoding rate, then prepares the packet stream for 8-VSB modulation by encoding the bit stream to eight discrete levels. The Trellis encoded packet stream 180 is then routed to an 8-VSB modulator (Not Shown) for transmission of the HDTV signal.
FIG. 2 depicts an exemplary prior art trellis encoder, shown in more detail. A precode sequence is generated by an interference filter pre-coder 210 from the X2 input, which is the most significant part of a randomized packet stream containing HDTV information. The filter output is then routed to trellis encoder 215 where it is fed through without any further processing to the Z2 output. The X1 input represents the least significant part of a randomized packet stream containing HDTV information. The trellis encoder feeds X1 through to the Z1 output. The Z0 output is a function of the X1 input and historical (two 12 symbol delay blocks, D) X1 values. The three bit Z2Z1Z0 combination is mapped by an 8-level symbol mapper 220 to the R channel symbols representing the eight discrete levels for 8-VSB modulation. It should be noted that in addition to utilizing a 2/3 encoding rate with one bit pre-coded so as to maintain an effective encoding rate of 1/2, the existing A/53 prior art, i.e., legacy trellis encoder for HDTV transitions through 4 states, i.e., the A/53 trellis encoder is a four state machine. A complete discussion of 4 state trellis encoding may be found in U.S. patent to Csajka et al (U.S. Pat. No. 4,077,021), and is incorporated herein by reference.
Under ideal reception conditions, i.e., no multipath signal interference, atmospheric or other type interference, the above known transmission encoding scheme is adequate to deliver the payload, i.e. HDTV signal, to a receiver with no degradation in picture quality. However, terrestrial, i.e., on-air television broadcasting may encounter many types of interference and disturbance between the transmission system and the reception system. Signal strength loss and interference contribute to a minimum threshold of visibility (TOV), typically measured in decibels (DB) also measured as carrier to noise ratio (CNR) above a noise/interference level, under which the HDTV signal simply cannot be recovered.
While the above discussed prior art system is beneficial in the capability to transmit an HDTV signal over an existing legacy on air channel such as the 6 MHz wide channel discussed above, there remains the drawback of maintaining a relatively high TOV over interference and noise for successful reception of the HDTV signal. In real world applications, however, HDTV signal attenuation and degradation due to a variety of factors, such as those discussed above, are likely, when encountered, to cause an 8-VSB signal to fall below its required TOV, resulting in disruption of reception at an HDTV receiver. While legacy analog systems may degrade slowly under interference, i.e., the picture may gradually fade away into a noise signal (snow), the digital HDTV signal will suddenly freeze at the below TOV level. While a robust data stream solves the problem of reducing the TOV in order to maintain HDTV viewing under less than ideal conditions, there remains the problem of maintaining compatibility with legacy, i.e., prior art HDTV transmission and reception systems. The prior art simply has a need for a more robust and backwards compatible HDTV system.